CHAPTER 9: THEORY OF OPERATION OVERVIEWL60 LINE PHASE COMPARISON SYSTEM – INSTRUCTION MANUAL 9-159Figure 9-9: Typical power line carrier arrangementIn the case of ON-OFF power line carrier channels, the operating frequencies of the equipment at all terminals of theprotected line are generally the same. Thus, a signal transmitted from any terminal is received at all terminals. This is not anecessary requirement for using this kind of equipment. Rather it is desirable because the protection schemes that useON-OFF channels can accommodate a single frequency arrangement and this conserves the carrier spectrum.When frequency-shift equipment is used over power line carrier, the frequencies of each transmitter on the line must bedifferent from all the others on the same line. For example, if the communication equipment in the Vector Relationships ina Two-Terminal Faulted Line (A-to-G) figure is of the frequency-shift type, then the transmitter at the left end must operateat the same frequencies as the receiver at the right end. Also, the right end transmitter and left end receiver must operateat the same frequencies while the frequencies of the two transmitters must be different. This is necessary because withfrequency-shift equipment the transmitters associated with a given line protection scheme are not all generally sendingthe MARK or the SPACE frequencies at the same time. Thus, if a receiver were able to receive more than one transmitter, itcan be simultaneously receiving a MARK signal from one and a SPACE signal from another.This does not result in a workable protection scheme. When power line carrier channels are used, significant losses arepresent in the coupling equipment and the line itself. Depending on these losses and the ambient noise on the line, thetransmitter power required can vary from about 1 to 10 watts and even more in extreme cases.Consider an ON-OFF tripping type of scheme as defined by the following figure. For a moment assume that FDL and NOT1do not exist in the logic. During an internal fault, the currents out of the mixing (or sequence) networks at both ends of theline are in phase with each other so that the outputs of the SQ AMP are in phase at both ends of the line. The transmittersat both ends of the line are keyed on during the same half cycles that their associated SQ AMPs are attempting to trip viaAND1. Thus, the receivers supply the bottom input to AND1, and tripping takes place when FDH operates to provide thethird input.
